1. Field of the Invention
This invention relates generally to a low frequency mechanical resonator and, more particularly, to a piezoelectric resonator filter capable of being fabricated as part of an integrated circuit on a single chip and being capable of selecting relatively low frequencies.
2. Discussion of the Related Art
Analog integrated circuits, such as amplifiers, mixers, and the like, configured on a single integrated chip, such as a monolithic microwave integrated circuit (MMIC), are rapidly replacing larger more cumbersome pieces of electronic hardware which perform the same function. However, narrow band analog filters and resonators, that have bandwidths less than 2%, have as of yet not been developed so as to be effectively integrated on a single chip as part of an integrated circuit. Devices such as crystal resonators, surface acoustic wave (SAW) resonators and SAW filters, known to those skilled in the art, provide outstanding performance as discrete devices, but are too large to be integrated on a single chip. Consequently, development of resonators on an integrated circuit level that have desirable performance is currently under way.
Semiconductor bulk acoustic resonators (SBAR) are emerging as one viable device for use as a resonator and filter in integrated circuits. Such a device is disclosed in the paper R. B. Stokes et al., "X-Band Thin Film Acoustic Filters on GaAs," presented at the 1992 IEEE MTT Symposium. FIG. 1 shows a typical prior art SBAR circuit element 10 which includes an SBAR 12 positioned on a substrate 14. The SBAR 12 includes a thin piezoelectric layer 16 formed between top and bottom metal film electrodes 18 and 20. The SBAR 12 is configured on the substrate 14 over a backside via recess 22 etched in the substrate 14. In a typical case, the substrate 14 is GaAs, having a thickness of about 4 mils, the piezoelectric layer 16 is aluminum nitride (AlN), and the electrodes 18 and 20 are aluminum having a thickness of about 1000 .ANG.. The recess 22 adjacent the SBAR 12 is square where each side is approximately 300 microns. By applying a suitable alternating current to the top and bottom electrodes 18 and 20 by leads 24 and 26, respectively, the piezoelectric layer 16 will alternately expand and contract in association with the alternating current so as to generate a longitudinal wave within the piezoelectric layer 16. The piezoelectric layer 16 will resonate at a narrow band frequency. The recess 22 enables the SBAR 12 to vibrate without losing acoustic energy to the GaAs substrate 14. By stacking SBAR piezoelectric membranes in an SBAR structure, a two-port filter can be developed for producing a narrow band output signal.
The SBAR circuit element 10 has been shown to be effective as a resonator for selecting narrow bandwidth frequencies within the range of about 500 MHz to about 20 GHz. However, because the SBAR 12 resonates as a result of longitudinal waves travelling in the thickness direction of the piezoelectric layer 16, this device is impractical at frequencies lower than 500 MHz. For output frequencies below 500 MHz the piezoelectric film required to get a desirable resonance becomes very thick and difficult to fabricate. Therefore, acoustic resonators and filters at these lower frequencies cannot be effectively fabricated as part of an integrated circuit.
What is needed is a resonator and filter which can be fabricated as part of an integrated circuit, and which selects narrow bandwidths at relatively low frequencies. It is therefore an object of the present invention to provide such a resonator and filter.